Bushmaster Carbon 15 problem

[BuckKlier]

I have a Bushmaster Carbon 15 that the rail for mounting the scope has come loose again from the polymer upper, Bushmaster replaced the upper once under warranty (one Year) now it has come loose again, anyone else have this problem? It is now out of warranty. I will be installing a metal upper on it.

[Ironbear]

Would that have a metal rail bonded to the composite upper? If so, the differing coefficients of thermal expansion will wreak havoc during thermal cycles.

Adhesive selection and surface prep are critical, but bonding metal to composites is not trivial.

[JeffB]

They're screwed on with a single recessed flat head allen screw at the front of the rail, there's no gluing involved. The back of the "rail" lifts easily with a fingernail. Mine moves around a little too.

It's not from the heat, a single mounting screw is far from ideal. "Composite" and "polymer" are way-overused buzz words in this and many other industries. It's simple glass-filled nylon - plastic. But "plastic" doesn't sound so sexy as a selling point I guess. It wouldn't be too tough to add another screw in the back if you have the equipment, or just drill and tap carefully by hand. I'll be doing that to mine on the Bridgeport at work when I get time to run in on a weekend.

This nylon will only expand about .003-.004" per inch of length from room temp to around 180F. And you might have other problems with parts alignment at that point anyway. I'm an injection molder, this is easy stuff to cover.

[BuckKlier]

They're screwed on with a single recessed flat head allen screw at the front of the rail, there's no gluing involved. The back of the "rail" lifts easily with a fingernail. Mine moves around a little too.

It's not from the heat, a single mounting screw is far from ideal. "Composite" and "polymer" are way-overused buzz words in this and many other industries. It's simple glass-filled nylon - plastic. But "plastic" doesn't sound so sexy as a selling point I guess. It wouldn't be too tough to add another screw in the back if you have the equipment, or just drill and tap carefully by hand. I'll be doing that to mine on the Bridgeport at work when I get time to run in on a weekend.

This nylon will only expand about .003-.004" per inch of length from room temp to around 180F. And you might have other problems with parts alignment at that point anyway. I'm an injection molder, this is easy stuff to cover.

The screw has come loose on mine, and there is a small crack from the screw hole to the bushing for the gas tube. I bought a "Blem." stripped upper from Palmetto State Armory for $30, and a barrel nut assy. (The plastic upper takes a bigger one)

[PhilaBOR]

Not sure what your setup looks like, but you may want to use a Helicoil: buy a kit that includes a drill, the Helicoil and installation tool. Or just put the money towards a 7075 aluminum upper.
Based on the name, I always assumed the carbon 15 was made from carbon fiber reinforced nylon, not glass, but don't know. IIRC, carbon nylon is 30ksi yield, glass nylon is 20ksi.

[yuppiejr]

The Anderson lightweight sport upper (no forward assist or ejection port door) only costs $45 - http://www.andersonrifles.com/product/l ... abilities/. I'd consider swapping uppers to be a better use of time and money vs. drilling/tapping more screws into a plastic upper or investing the money into helicoil kit and hoping the process doesn't further weaken an already marginal material for an AR upper receiver. The 7075-T6 aluminum used in most AR15 upper forgings has a yield strength north of 70,000 PSI vs 20,000 for the plastic used in the original C15's which gives you a lot more margin of error if something goes wrong.

The aluminum upper is stronger, lighter, and a more stable platform on which to mount optics of any sort.